Activities of unsupported second transition series metal sulfides for hydrodesulfurization of sterically hindered 4,6-dimethyldibenzothiophene and of unsubstituted dibenzothiophene

The applicability of transition metal sulfides (TMS) from the second transition series in deep hydrodesulfurization (HDS) was examined and compared to that of a traditional, supported CoMo/Al2O3 catalyst. Sulfides of Nb, Mo, Ru, Rh and Pd were studied for HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-Me2DBT). Measurements were carried out with unsupported TMS samples at different temperatures and H2S partial pressures. The trend in DBT HDS activities agreed quite well with those found by previous authors. It was furthermore found that the activities of the metal sulfides towards the sterically hindered molecule 4,6-Me2DBT closely followed those for DBT. This is somewhat surprising since the direct sulfur abstraction route was of major importance for DBT while the prehydrogenation route, in which ring-hydrogenation in the DBT skeleton precedes desulfurization, was prevalent for 4,6-Me2DBT. This suggests that common steps are involved in the two routes. For the unsupported metal sulfides, ring-hydrogenated but not desulfurized DBT and 4,6-Me2DBT products were found in much larger amounts than for supported and promoted MoS2-based catalysts. This can be rationalized as being due to a relatively higher hydrogenation/desulfurization selectivity ratio for the different transition metal sulfides. Inhibition by H2S was found to be most pronounced near the center of the transition series.